Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires a gas ion bombardment of the target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
In addition to the use of an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target surface thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e., consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target race-way.
In conventional target cathode assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-known dispositions in order to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
In order to achieve good thermal and electrical contact between the target and the backing plate, these members are commonly attached to each other by use of soldering, brazing, diffusion bonding, clamping, epoxy cements, etc.
Some sputtering systems use targets that are not bonded to the water cooling assembly. Instead they fit into a water jacket of a water cooling assembly with a close tolerance. When sputtering starts, these targets heat up and expand to contact the water cooling assembly and are cooled for further sputtering so that the temperature of the sputtering target does not increase significantly. This method works well for target materials that are reasonably strong.
Some sputtering target materials are very weak or have very low yield strength. Pure aluminum is an example. When pure aluminum expands against the water cooling assembly, it is so weak that it deforms and shrinks during sputtering. As it gets smaller, it runs hotter and hotter, resulting in poorer sputter performance. Making these targets stronger would be very desirable, but when pure aluminum is required for the sputtering material, no alloy elements can be added to strengthen the material.
In U.S. Pat. No. 4,385,979 (Pierce et al ) fragile sputter target materials are provided with a retaining cup or band member (see FIGS. 3a-3c) and a bonding means between the target and retaining cup or band. The retaining member helps to alleviate target warping, cracking and/or overheating problems that may otherwise occur upon usage. The present invention provides improvement in methods of fabricating target/retainer cup or band assemblies of the type shown in Pierce et al.
TiG welding of a target to a backing plate structure is disclosed in U.S. Pat. No. 5,009,765 (Qamar et al--of common assignment herewith), although the target disclosed in this patent is not cooled via expansion of the target sidewall into a surrounding water jacket but rather is cooled by direct impingement of cooling water on the bottom surface of the target.
Roll bonding of a target to a backing plate is disclosed in U.S. Pat. No. 5,032,468 (Dumont et al) with the patent to Korb, U.S. Pat. No. 4,752,335 teaching target manufacturing methods involving cold pressing of the requisite powders followed by densification of the material by repeated working at temperatures below the lowest of the melting points of the individual components. This treatment occurs under conditions of material flow and cold welding.
The Pouliquen U.S. Pat. No. 5,087,297 discloses forging of a target to develop a desired grain for subsequent use in magnetron sputtering.
Other patents which are generally of interest to the field of friction welding, but not cathodic sputtering include U.S. Pat. No. 3,693,238 (Hoch et al); U.S. Pat. No. 3,452,421 (Cheng et al) and U.S. Pat. No. 3,998,373 (Jones et al). Of background interest to sputtering procedures and structures with specific reference to magnetron sputtering, U.S. Pat. No. 4,457,825 (Lamont, Jr.) is mentioned.